Diesel engines are substantially different from the standard 4 or 2 cycle, sparked ignition internal combusion engines. The diesel engine does not have a sparking device such as a standard spark plug. Fuel is ignited when fuel and hot compressed air are mixed in the engine cylinder(s). For this ignition to occur efficiently, the engine must be brought to a temperature at or above a given minimum operating temperature, i.e. a cold diesel engine will not achieve ignition and run efficiently.
A preferred method for heating a diesel engine prior to initial startup is to use electric "glow plug" heaters. These heaters serve to bring the diesel engine up to an efficient operating temperature before the engine is started. Ideally glow plug heaters will rapidly bring a diesel engine up to a desired starting temperature in a "pre-glow" period. After the engine has started, the glow plugs will go into an "after glow" period where they will operate sufficiently long to maintain desired engine temperature until engine self-heating reaches an efficient sustain point. The glowplugs also enable the engine to run smoothly during an initial idle and minimize emission of white smoke due to incompletely burned fuel. Once an engine can sustain its operating temperature, the glow plug is turned off.
U.S. Pat. No. 4,882,370 to Arnold et al shows a solid state microprocessor controlled device for regulating certain aspects of glow plug performance. The Arnold circuitry adjusts the duty cycle of glow plugs as a function of temperature, regulates preglow function, and detects undesirable short circuits and open circuits for implementing a disable function. U.S. Pat. No. 4,300,491, to Hara et al., achieves a variable time control of the preglow period by means of a plurality of transistors and diodes. Van Ostrom, U.S. Pat. No. 4,137,885 describes means for cyclicly interrupting a glow plug energizing circuit when a maximum temperature is reached. Cooper, U.S. Pat. No. 4,312,307 describes circuitry for control of the duty cycle of glow plugs by means of heat-sensitive switches.